The present invention relates to an RF amplifier, and more particularly to an RF amplifier which is equipped with a dual gate field effect transistor, i.e. an FET.
In the antenna tuning circuit for a VHF electronic tuner according to the prior art, for example, a dual gate MOS FET is so constructed that its first gate is fed with a signal voltage whereas its second gate is fed with an automatic gain control (AGC) voltage. The first gate is typically impressed with a predetermined bias voltage, as shown in FIG. 1, wherein reference numerals 1, 2 and 3 appearing in FIG. 1 indicate the dual gate MOS FET, the first gate and the second gate thereof, respectively. Numerals 4, 5 and 6 indicate a variable capacitance circuit unit, a band switching circuit unit and a bias voltage supply unit, respectively. Numerals 7, 8 and 9, 10, 11 and 12, 13 to 16, and 17 indicate a variable capacitance diode, two coils, a switching diode, two condensers, four resistors, and a biasing high resistor, respectively. Numeral 18 indicates a tuning voltage terminal. Numerals 19, 20 and 21 indicate a high-band selecting voltage terminal, a low-band selecting voltage terminal, and a biasing voltage supply terminal, respectively. Moreover, numerals 22 and 23 indicate a coupling condenser and a resistor, respectively.
Upon the selection of a high band, the terminal 19 is impressed with a positive voltage. In this case, a current biasing the switching diode 10 in the forward direction flows through the resistor 14, the diode 10, the coil 9 and the resistor 15. As a result, since the diode 10 is forwardly biased, the coil 9 is short-circuited in view of the high frequency through the diode 10 and the condenser 11 so that the coil 8 and the condenser 11 coact with the variable capacitance circuit unit 4 thereby to be tuned to a channel within the high band. Upon the selection of a low band, on the contrary, the terminal 20 is impressed with a positive voltage. In this case, since the current flows through the resistors 16 and 15, the diode 10 is biased in the backward direction through the coil 9 so that the diode 10 is held at its nonconductive state while allowing no current to pass therethrough. As a result, the coils 8 and 9 and the condenser 12 coact with the variable capacitance circuit unit 4 so that they are tuned to a channel within the low band.
In the conventional antenna tuning circuit having the construction thus far described, a predetermined bias voltage is supplied, as shown, from the terminal 21 through the resistor 17. As a result, in case the frequency difference between the high and low bands is high, as in the VHF band of the USA, for example, the performane of the AGC becomes different between the high and the low bands.